Paint Flow Control Project, Chapter 3.23, Good Progress

Proudly wearing my Berkeley sweatshirt
with my wife Heide's painting in the background.

The funding really did come through from Vivid Inc (VividInc) of Santa Clara and Universal Electronics (UEI) down in Santa Ana. Vivid needs a paint flow controller at their coating and painting facility and UEI needs paint flow controllers for their overseas manufacturing facilities. Initially they want only four systems. If these work out for them, they will need a few more, maybe about eleven more. So there's not a big need for our systems to be sure, but the controllers should be big cost savers as the conductive paint is quite expensive and they want to use just enough paint for the RF shielding.

The paint flow control system comprises the controller, a flowmeter to measure the paint flow rate, a paint pump, a paint gun and a robot to steer the paint gun and turn it on and off. The flowmeter outputs a 4-20 mA current for the flow rate and the pump is controlled by an I/P converter, requiring a 4-20 mA current. I helped to choose the flowmeter and the I/P converter for the systems.

As I have been saying in previous posts, for the controller and User interface we decided to use a Beaglebone Black (BBB) single-board-computer (SBC) along with a 4D Systems (4D) touch-screen LCD, the 4DCAPE-70T (4DCAPE_70T). I used commercial SBC's for previous projects with K&H Finishing in San Jose and Enclosures Engineering of Fremont and recently had designed my own SBC with analog I/O, Ethernet and Serial Communication capability but Vivid and UEI wanted a commercial board for this project, preferring not to manufacture a board. So I had planned on using a BL2600 SBC from Digi International (BL2600) for the control and a notebook PC to communicate with the BL2600 and provide a GUI for the operator. However Thomas Nguyen, my cohort at Vivid, suggested using a touch-screen LCD panel instead of the PC in order to simplify the system for the User. I searched and found the BBB and the 4D Systems 4DCAPE, figuring that I could do the whole job with those two widgets and thereby greatly simplifying the control system. The BBB has an onboard Analog-To-Digital Converter (ADC) for measuring the current from a paint flowmeter and has a Pulse-Width_modulation output for providing a control signal to control the pump speed. It does need some interface circuitry to convert the 4-20 mA current from the flowmeter to voltage, a resistor, and an operational amplifier and transistors to convert the PWM output signal to a 4-20 mA current to control the pump. So I got to design a little board for those purposes, using Kicad (Kicad), a freebie CAD software package. Kicad has its idiosyncrasies as all CAD programs do, I suppose, but I have found it to be usable and the Kicad user group is quite helpful for beginners (Kicad Users).

I had been working on a simulation of the system using Python (Python) and decided to continue using Python in the BBB for the control code and also for the GUI. I like using Python since it's a faster development process, not requiring the usual compile and link and do over and over until you get it right. Python is an interpreter so the programs run slower than a C or C++ or C# program but the slower speed is not a problem with the paint flow controller since the process is very slow. In order to produce a GUI for the operator, I used Tkinter. That has been something of a problem, giving me fits with a Tkinter error that I mentioned in the last post.  However, my code can be run by connecting a keyboard to the BBB thru its USB interface and starting the code with "sudo python myProg.py." But certainly that's not adequate. I can't require the users to have to type in a command to get myProg.py up and running since those guys or gals are in a manufacturing environment and probably in Asia. They would like a simple button to press. If possible, it would be nice for myProg.py to start up on booting the BBB.

So I set about finding ways to start my Python code with a button press or touching a "button" on the 4DCAPE screen. I found that there are three ways to do that:

1. systemd
2. cron
3. icon on desktop, Debian or other Linux.

Lots of info on systemd and cron is available with an Internet search, so I won't repeat it. I tried and tried to get either of them to start up my SimB.py Python program but wasn't able to do so. I think the problem is the Tkinter error that I mentioned above. However, with the help from a post by Cyril Laury in this askubuntu post (Icon StartupDesktop Icon), I was able to put an Icon on the Debian desktop and have it call a bash script to start my program. Here are the scripts.

paintflow.desktop
[Desktop Entry]
Version=1.0
Exec=sudo /home/debian/Desktop/startcontrol.sh
Name=Paint
Comment=Start up SimB.py
Encoding=UTF-8
Terminal=false

Type=Application

startcontrol.sh:
#!/home/bash
#sudo /bin/bash clock_init.sh
sudo /usr/bin/python2.7 /home/debian/Desktop/SimB.py

Many thanks to Cyril Laury.

My next software tasks were:

1. record data on a USB flash drive plugged into the BBB,
2. add a Real Time Clock (RTC) module to the BBB,
3. add software for the RTC,
4. add an Idle mode to the paint flow control when the paint gun was turned off.

Then there are the hardware tasks: 

1. fabricate the Analog I/O boards and get them loaded,
2. design the hardware system, purchase power supplies and other parts, and
3. get the hardware installed in the Protocase enclosures to complete the hardware tasks.

So I still have much to do and will report on those tasks in future posts.

So, that's all for now and back to work.

John